1. Field of the Invention
The present invention relates to a receiver for receiving a signal transmitted from a transmitter via a transmission line, and a signal transmission system for transmitting a signal via a transmission line.
2. Description of the Related Art
In a conventional signal transmission system, a terminal resistor is connected to one end or both ends of a transmission line. The resistance value of the terminal resistor is set to about 50 ohms so as to match with the impedance of the transmission line (about 50 ohms). In such a signal transmission system, the value of a signal being transmitted (i.e., xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d) is determined by the direction or the current value of the current flowing through the terminal resistor.
For example, consider a case where a 1.5 V terminal power supply and a transmission line are connected to each other via a terminal resistor having a resistance of 50 ohms. In such a case, when a current flows into the transmission line from a driver of a 3.0 V power supply voltage, the transmission line has a potential (xe2x80x9cHxe2x80x9d potential) which is higher than the 1.5 V terminal power supply by a voltage increase. The voltage increase is determined based on the current flowing into the 1.5 V terminal power supply and on the terminal resistance of 50 ohms. The H potential represents a transmitted signal having a value xe2x80x9c1xe2x80x9d, for example.
Conversely, when the driver draws a current from the 1.5 V terminal power supply toward a point at the terminal potential, the transmission line has a potential (xe2x80x9cLxe2x80x9d potential) which is lower than the 1.5 V terminal power supply by a voltage decrease. The voltage decrease is determined based on the current flowing out of the 1.5 V terminal power supply and the terminal resistance of 50 ohms. The L potential represents a transmitted signal having a value xe2x80x9c0xe2x80x9d, for example.
In such a conventional signal transmission system, a change in current of 10 mA is required for obtaining a change in signal on the transmission line of 500 mV.
The above-described conventional signal transmission system has the following disadvantages: 1) the change in signal on the transmission line is determined by the voltage decrease (or the voltage increase) which is caused by the terminal resistor. Therefore, in order to increase the change in signal on the transmission line, it is necessary to increase the amount by which the voltage is decreased (or the amount by which the voltage is increased) which is caused by the terminal resistor; and 2) the resistance value of the terminal resistor is determined so as to match with the impedance of the transmission line. Therefore, the resistance value of the terminal resistor cannot be increased to increase the change in signal on the transmission line.
Due to the disadvantages 1) and 2), the only way to increase the change in signal on the transmission line is to increase the change in the current flowing through the transmission line. In order to increase the change in the current flowing through the transmission line, the conventional signal transmission system determines the value of the transmitted signal by switching the direction of the current flow along the transmission line or by switching between conducting a current through the transmission line and conducting no current therethrough.
However, such a large change in the current flowing through the transmission line may disturb the waveform of the transmitted signal. This is because the transmitted signal may be influenced by an induced potential (dV=xe2x88x92L*dI/dT), which is determined by the product of the inductance value (L) of the transmission line and the rate of change per unit of time (dI/dT) in the current flowing through the transmission line.
Such a large change in the current flowing through the transmission line may disturb the waveform of the transmitted signal also in a differential type signal transmission system in which two transmission lines are short circuitted via a resistor.
According to one aspect of this invention, a receiver for receiving a signal transmitted from a transmitter via a transmission line includes: a current control section for allowing a current to flow into the current control section from the transmission line or flow from the current control section to the transmission line, wherein the current has an amount which varies according to a logical level of the signal and flows in a same direction irrespective of the. logical level of the signal; and a determination section for determining the logical level of the signal based on the amount of current which has flowed from the transmission line into the current control section or based on the amount of current which has flowed out from the current control section to the transmission line.
In one embodiment of the invention, the current control section includes: a constant current supply section for supplying a constant current; and a difference current compensation section for compensating for a difference current having an amount which represents a difference between the amount of current which has flowed in from or flowed out to the transmission line and an amount of the constant current.
In one embodiment of the invention, the determination section determines the logical level of the signal based on a direction in which the difference current flows.
In one embodiment of the invention, the logical level of the signal xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d is represented by the amount of current flowing through the transmission line.
In one embodiment of the invention, three or more different logical levels of the signal are represented by the amount of current flowing through the transmission line.
According to another aspect of this invention, a signal transmission system for transmitting a signal via a transmission line includes: a current supply section for supplying a current to the transmission line, wherein the current has an amount which varies according to a logical level of the signal and flows in a same direction irrespective of the logical level of the signal; a current control section for allowing the current to flow into the current control section from the transmission line or flow from the current control section to the transmission line irrespective of the amount of the current supplied from the current supply section to the transmission line; and a determination section for determining the logical level of the signal based on the amount of current which has flowed from the transmission line into the current control section or based on the amount of current which has flowed out from the current control section to the transmission line.
In one embodiment of the invention, the current control section includes: a constant current supply section for supplying a constant current; and a difference current compensation section for compensating for a difference current having an amount which represents a difference between the amount of current which has flowed in from or flowed out to the transmission line and an amount of the constant current.
In one embodiment of the invention, the signal transmission system includes a transmitter for transmitting the signal and a receiver for receiving the signal. The current supply section is included in the transmitter. The constant current supply section, the difference current compensation section and the determination section are included in the receiver.
In one embodiment of the invention, the signal transmission system includes a transmitter for transmitting the signal and a receiver for receiving the signal. The current supply section and the constant current supply section are included in the transmitter. The difference current compensation section and the determination section are included in the receiver.
In one embodiment of the invention, the determination section determines the logical level of the signal based on a direction in which the difference current flows.
In one embodiment of the invention, the logical level of the signal xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d is represented by the amount of current flowing through the transmission line.
In one embodiment of the invention, three or more different logical levels of the signal are represented by the amount of current flowing through the transmission line.
According to still another aspect of this invention, there is provided a receiver for receiving a signal transmitted from a transmitter using a first transmission line and a second transmission line. A current flows through the first transmission line and the second transmission line, wherein the current has an amount which varies according to a logical level of the signal and flows in a same direction irrespective of the logical level of the signal. The receiver includes: a difference current compensation section for compensating for a difference current having an amount which represents a difference between the amount of current flowing through the first transmission line and the amount of current flowing through the second transmission line; and a determination section for determining the logical level of the signal based on the difference current.
In one embodiment of the invention, the logical level of the signal xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d is represented by the amount of the difference current.
In one embodiment of the invention, three or more different logical levels of the signal are represented by the amount of the difference current.
According to still another aspect of this invention, there is provided a signal transmission system, including a transmitter, a receiver, and a first and second transmission line for connecting the transmitter and the receiver to each other, for transmitting a signal from the transmitter to the receiver using the first transmission line and the second transmission line. The transmitter includes a current supply section for supplying, to the first transmission line, a current having an amount which varies according to a logical level of the signal. The receiver includes a determination section for determining the logical level of the signal based on a difference current having an amount which represents a difference between the amount of current flowing through the first transmission line and the amount of current flowing through the second transmission line. The first transmission line and the second transmission line are formed by folding back a single transmission line.
In one embodiment of the invention, the receiver further includes a difference current compensation section for compensating for the difference current.
In one embodiment of the invention, the logical level of the signal xe2x80x9c0xe2x80x9d or xe2x80x9c1xe2x80x9d is represented by the amount of the difference current.
In one embodiment of the invention, three or more different logical levels of the signal are represented by the amount of the difference current.
Functions of the present invention will be described below.
According to the present invention, a current flows in the same direction along a transmission line irrespective of a logical level of the transmitted signal. Thus, it is possible to reduce the change in the signal on the transmission line. This assists in reducing the disturbance in the waveform of the transmitted signal. Moreover, substantially all of a variable amount of current which varies according to the logical level of the transmitted signal is absorbed from or flows out to the transmission line by a current control section. Thus, substantially no signal is reflected at an end of the transmission line. This eliminates the need to provide a terminal resistor on the receiver side of the transmission line.
Where a transmitter for transmitting a signal and a receiver for receiving the signal are remote from each other, the ground potential of the transmitter and that of the receiver are often different from each other. Even when the ground potential of the transmitter is the same as that of the receiver, the power supply potential of the transmitter and the ground potential of the receiver may be different from each other due to a decrease in the power supply potential. Moreover, there are cases where the power supply potential of the transmitter and the power supply potential of the receiver are different from each other, such as when the power supply of the transmitter is a 3.3 V power supply while the power supply of the receiver is a power-saving 1.5 V power supply.
In such cases, a current can flow more easily in one direction than in the opposite direction along the transmission line. This is because a current by nature flows from a higher voltage point toward a lower voltage point. According to the present invention, it is possible to design a signal transmission system such that the direction of the current flow along the transmission line coincides with the direction in which a current can flow more easily.
The present invention provides advantages as described above by employing the structure where a current flows in the same direction along the transmission line irrespective of the logical level of the transmitted signal.
Thus, the invention described herein makes possible the advantages of (1) providing a receiver and a signal transmission system in which a change in a current flowing through a transmission line is reduced; and (2) providing a receiver and a signal transmission system which eliminate a terminal resistor connected to a transmission line.